Following written copy from the Chicago Architecture Foundation.
Architects have always been tasked with designing tall buildings to resist wind loads. But as buildings continued to grow taller and taller, new solutions were required. When this 110-story tower was designed, architect Bruce Graham and structural engineer Fazlur Khan of Skidmore, Owings & Merrill (SOM) utilized the bundled tube system to address structural problems never before seen at this scale. Their use of the innovative system ushered in a whole new era of skyscrapers.
The tower is comprised of a cluster of nine tubes connected together to act as a single unit. The tubes support one another, strengthening the structure as a whole. Meanwhile, the variations in tube height disrupt the force of the wind.